Evolution of defects in copper deformed by high-pressure torsion

Abstract

Lattice defects in Cu deformed by high-pressure torsion (HPT) were investigated by positron annihilation spectroscopy (PAS) combined with transmission electron microscopy, X-ray diffraction (XRD) and Vicker’s microhardness (HV) measurements. The evolution of the microstructure during HPT processing was studied on samples subjected to various numbers of HPT revolutions using pressures of 2 and 4 GPa. Since strain in torsion deformation increases with the radial distance from the center of rotation, one can expect a non-uniform microstructure across the sample diameter. To examine this, HV was measured at various distances from the center of the HPT-deformed sample and the microstructure at the center was compared with that at the periphery. It was found that HPT-deformed Cu contains a high density of dislocations and also small vacancy clusters formed by the agglomeration of deformation-induced vacancies. The center of the sample exhibits coarser grains, a slightly lower density of dislocations and smaller vacancy clusters compared to the periphery. The dislocation density and concentration of vacancy clusters were evaluated from the combination of the PAS and XRD results. The theoretically estimated concentration of deformation-induced vacancies is of an order of magnitude comparable to that determined in experiment.